You don't want a prebuilt battery. If it's from China, it is mostly sand inside or fraudulent cells with a tenth or less of rated capacity.
If you absolutely must have a prebuilt pack, get one from New Trent. The quality and price are as good as it gets for this kind of thing.
A better option is to simply buy an enclosure for 18650 cells. These are the same cylindrical lithium cells which you'd find inside your laptop battery if you opened it up. This way you get to choose the cells you buy for it, and if one dies, you replace it with a fresh one instead of tossing the whole thing and buying a new one.
http://www.ebay.com/itm/5x-18650-Battery-Dual-USB-LCD-Backup-Display-Mobile-Charger-Power-Bank-LED-/181289093639Now, you need cells for it. Do not buy anything labeled Ultrafire, 90% of those are fakes. Buy brand names like Samsung or Panasonic. If you want the most energy storage possible today, go with the Panasonic NCR 18650b. It has 3100mah per cell (really 3100mah, I tested) aka about 12 and a half watt hours per cell. 5 of them loaded into that enclosure = 62.5 watt hours. Not too shabby.
http://www.amazon.com/Panasonic-NCR18650B-Rechargeable-Battery-Green-quantity/dp/B00CB8PGOE/ref=sr_1_2?ie=UTF8&qid=1404110851&sr=8-2&keywords=panasonic+ncr18650bThey are only sold in batches of 2 or 4, so you'll need to put in 3 under quantity on that page. Keep the extra one. Later if you want to buy 4 more, added to that 1 leftover it'll form a set of 5 spares you can swap in like you described when the first 5 are depleted.
The enclosure is also a charger and accepts the same small format USB input as an Android smartphone. So, with the right cable, you can easily charge this pack from a folding solar array. As a side note, they somehow confused the 1 amp and 2 amp ports. 1 amp is actually 2 amps and vice versa. If you need to charge something big like a tablet, use the port marked 1A.
